[@aws-appsync/eslint-plugin] missing rule: no-throw

Since `throw` statements are not allowed in AppSync JS, there should be a `no-throw` rule to detect and report any `throw` statements

The issue names the @aws-appsync/eslint-plugin but no files or tests; start by locating its existing rule implementations and test patterns. Add coverage for throw statements so the new no-throw rule reports them, and confirm the plugin tests pass.

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